1. Field of the Invention
The present invention relates to a phase-change optical recording medium in which phase change between a crystalline phase and an amorphous phase is reversibly caused by irradiation with a light beam so as to record information.
2. Description of the Related Art
(Principle of Phase-Change Optical Recording Medium)
The phase-change optical recording medium, comprising a phase-change optical recording film that permits reversible phase change between a crystalline phase and an amorphous phase upon irradiation with a light beam, is operated by the principle described in the following. In a write stage, a region irradiated with a light beam is heated to a temperature higher than the melting point thereof so as to be melted, followed by rapidly cooling the region to change the arrangement of the atoms in the region into an amorphous phase. In the erasing stage, a temperature in a region irradiated with a light beam is maintained for at least a prescribed period of time to fall within a temperature range from the crystallization temperature to the melting point. Then, where the initial state is crystalline, the crystalline phase is left unchanged. On the other hand, where the initial state is amorphous, the amorphous phase is crystallized. In a reading stage, utilizing the fact that the intensity of reflected light from the amorphous region differs from the intensity of reflected light from the crystalline region, the intensity changes of reflected light are converted into electric signals, and then the converted electric signals are subjected to analog-to-digital conversion so as to read out recorded information.
Incidentally, it is also possible to carry out read/write of information by utilizing a transition between a metastable crystalline phase such as a martensite phase and a stable crystalline phase or a transition between metastable crystalline phases, in addition to the phase change between the crystalline phase and the amorphous phase.
(Approaches to Improve Recording Density)
For increasing an amount of information that can be recorded in a single recording medium, i.e., for increasing recording capacity, it is conceivable to improve recording density by the two methods given below.
One method for improving the recording density is to reduce a pitch of the recording marks in the track direction. However, if the degree of size reduction proceeds, a region in which the pitch of the recording marks is made smaller than the size of the read beam is arrived at, with the result that it is possible for two recording marks to be included temporality in the read beam spot. Where the recording marks are sufficiently apart from each other, the read signals can be greatly modulated so as to make it possible to obtain signals having high amplitude. However, where the recording marks are positioned close to each other, signals having low amplitude are obtained, with the result that errors tend to be generated when the obtained signals are converted into the digital data.
The other method of improving the recording density is to reduce a track pitch. In this method, it is possible to increase the recording density while avoiding significant influence given by degradation in signal intensity caused by the reduction in the mark pitch noted above. However, this method gives rise to a problem of a so-called “cross-erase” that, in a region in which the track pitch is substantially equal to or smaller than the size of the light beam, data on a certain track is degraded while the adjacent track is undergoing writing or erasing.
The cross-erase is caused by the phenomenon that the recording mark is irradiated directly with the periphery of a laser beam on the adjacent track, and the phenomenon that the heat flow in the write stage flows into the adjacent track so as to elevate the mark temperature and, thus, to degrade the shape of the mark. It is necessary to overcome these problems for increasing the recording density of the phase-change optical recording medium.
(Approach to Achieve High-Speed Recording)
High-speed recording is another requirement for the phase-change optical recording medium. For example, where video signals can be recorded in a time shorter than an actual viewing time, it is possible to realize easily a so-called “time-shift function” which is referred to as a function of viewing previous scenes in dubbing a distributed recording medium or in recording a broadcasting program. One of the factors for inhibiting the high-speed recording in the phase-change optical recording is the problem that the data fails to be erased completely when the crystallization is performed by a laser beam having an erase level of a relatively low power in the overwriting stage, i.e., the problem of an insufficient erasure rate. Since a recording mark passes through a laser spot at high speed, the temperature of the recording mark fails to be maintained for a sufficiently long time to fall within a range within which crystallization can be achieved, with the result that the data fails to be erased completely.
An idea of arranging a GeN-based interface film in contact with a phase-change optical recording film for accelerating crystallization so as to increase the erasure rate is disclosed in “Acceleration of crystallization process by nitride interface layer”, Proceedings of The 10th Symposium on Phase Change Optical Information Storage, pp. 85-89, and in Japanese Patent Application KOKAI Publication No. 11-213446. However, according to the experiments conducted by the present inventors, it has been found that, in the phase-change optical recording medium having a GeN-based interface layer, a problem is generated in the write stage. The problem is based on the phenomenon that the peripheral portion of an initially melted region Im in the write stage is recrystallized, and an amorphous recording mark M is formed inside the recrystallized peripheral portion, as shown in FIG. 1. To be more specific, since it is necessary to melt a larger region in order to form a recording mark of a desired size, the cross-erase is to be promoted, with is a reverse effect in view of high-density recording. On the other hand, if the writing is performed with a laser power that is allowable in terms of the cross-erase, a problem is generated that the width of the recording mark to be formed is reduced so as to lower a carrier-to-noise ratio (CNR).
Such being the situation, it has been desired to develop a novel material for the interface film, which permits increased crystallization speed in erasing so as to overcome the problem in terms of the insufficient erasure rate and which also makes it possible to suppress the recrystallization of the melted region in writing.
(Increase in Recording Capacity by Dual-Layer Medium)
As another method for increasing the recording capacity, a method of superposing a plurality of information layers each containing a phase-change optical recording film is known. The particular method is disclosed in, for example, Japanese Patent Application KOKAI Publication No. 2000-322770. It should be noted that it is necessary for the first information layer positioned close to the light incident side to ensure at least about 50% of transmittance in order to prevent the light from being superfluously attenuated in accessing to the second information layer positioned remote from the light incident side. To this end, it is necessary to reduce the thickness of the recording film to about 5 to 8 nm. Since the thickness of the recording film is much reduced, the retention time required for the crystallization is made long, with the result that the recorded information fails to be erased completely in ordinary high-speed recording.
As a measure for overcoming the difficulty, it is disclosed that a method of substituting Sn for a part of the GeSbTe recording film is effective, in Proceedings of The 12th Symposium on Phase-change Optical Information Storage PCOS 2000, pp. 36-41. Also, it is disclosed that a method of substituting Bi, In, Sn or Pb for a part of the GeSbTe recording film is effective, in Japanese Patent Application KOKAI Publication No. 2001-232941.
Where the thickness of the recording film exceeds 15 nm, a temperature difference is generated between the upper portion of the recording film positioned closer to the reflection film and cooled at a high rate and the lower portion of the recording film. Therefore, crystal nuclei are generated on the upper portion of the recording film and the crystal nuclei grow so as to cause the entire recording film to be crystallized. However, where the thickness of the recording film is small, a sufficient temperature difference is not generated between the upper and lower portions of the recording film. Thus, for compensating the crystallization speed that has been lowered in accordance with the reduction in the thickness of the recording film, it is insufficient to adjust the composition of the recording film material, and it is necessary to arrange a film producing the effect of accelerating crystallization at the interface with the recording film. According to the Proceedings of The 12th Symposium on Phase change Optical Information Storage, it is effective to arrange, for example, a GeN interface film. However, it has been found as a result of research conducted by the present inventors that, in the combination of a thin recording film having a thickness of about 5 to 8 nm and a conventional interface film such as the GeN film, the cross-erase is generated to degrade read signal characteristics, with the result that errors are generated frequently in converting the read signals into digital data. Particularly, the signal characteristics are prominently degraded in the case where the thickness of the GeN interface film is smaller than about 5 nm.
On the other hand, the reflection film serves to cool the recording film that has been heated by the absorption of the recording light. However, since it is necessary to decrease the thickness of the reflection film in the semi-transparent information layer (the first information layer), the cooling function of the reflection film becomes insufficient, with the result that the read signal characteristics are degraded. As a measure against the difficulty, an idea of forming a heat sink film on the reflection film is disclosed in, for example, Japanese Patent Application KOKAI Publication No. 2000-222777. Also, it is reported that signal characteristics are improved in the case of arranging a heat sink film having a thickness of about 100 nm and made of AlN having very high heat conductivity, in Proceedings of ISOM/ODS 2002, pp. 234-236. However, it has been found as a result of research conducted by the present inventors that recording sensitivity is degraded in the case of using a material having very high heat conductivity such as AlN for a heat sink film.
As described above, in the first information layer (semi-transparent information layer) of the dual-layer single-sided phase-change optical recording medium, it is necessary to make both of the recording film and the reflection film thinner than those of the ordinary medium. Such being the situation, it has been desired to develop an interface film permitting a high CNR and an erasure rate while maintaining the cross-erase to a low level and to optimize the thermal characteristics of the medium including those of the interface film and the heat sink film.